Previous single feed point, dual frequency, circularly polarized, microstrip antenna designs required 2 microstrip feed networks for antenna arrays. Each feed network would be connected to a set of single frequency, circularly polarized, microstrip elements. The feed networks would be positioned back-to-back to achieve a single feedpoint design. The dual feed network approach described requires a diplexer at the common feedpoint to prevent detuning of one feed network by the other feed network. Typically, the diplexer will add approximately 0.5 dB loss to a dual band, circularly polarized, antenna system at L Band. The dual feed network approach requires space for the diplexer and enough clearance between the diplexer and adjacent feed network lines to prevent excessive coupling between the diplexer and adjacent feed networks. Excessive coupling between the diplexer and feed networks would result in a detuned diplexer and higher loss feed networks.
U.S. Pat. No. 4,692,769 to the same inventor, entitled Dual Band Slotted Microstrip Antenna, disclosed instantaneous dual band operation in a slotted microstrip radiating element. The two resonances are perpendicularly polarized and may be separated by as much as a 2:1 ratio.
U.S. Pat. No. 4,766,440 to the same inventor, entitled Triple Frequency U-Slot Microstrip Antenna, discloses an antenna with either triple frequency operation or dual frequency operation in which one frequency is circularly polarized and the other frequency is elliptically polarized.
The subject invention, in contrast, is a circularly polarized twin element design that interconnects two pairs of antenna elements in a configuration that suppresses parasitic resonances and permits incorporation into a closely spaced linear array. Any circularly polarized microstrip element can be used in this twin element configuration.